Solder paste is a material used in the manufacture of printed circuit boards to connect surface mount components to pads on the board. It is also possible to solder through hole pin in paste components by print solder paste in/over the holes.

I have a board with the above problem from our PCB supplier. After they applied the solder mask and run the PCB's to HASL process, the solder mask bubbled on one side of the PCB making the solder mask thickness higher and PCB looking awkward.

They said they are pushing in solder mask inside the holes during my conversation with them...what is that "pushing in solder mask inside the via" process? is this common way of applying solder mask onto 12.0 to 25.0 mils via holes?

What are the possible causes of the solder mask bubbling during HASL process?